It is sought more particularly here below in this document to describe problems existing in a wireless seismic data acquisition system. The present disclosure of course is not limited to this particular application but is of interest for any technique for providing synchronization in a data acquisition system that has to cope with closely related or similar issues and problems.
A first known solution is described in U.S. Pat. No. 8,228,757B2, entitled “Synchronization of modules in a wireless array”. This document discloses a method for synchronizing a wireless data acquisition module in a seismic array. The method comprises:                receiving at the wireless data acquisition module a time reference event from a separate component of the seismic array (as shown in FIG. 3 of this document, the time reference event provides a reference clock, coming either from a GPS receiver connected to the wireless data acquisition module, or from a radio event);        determining a delay value associated with the received time reference event, wherein the delay value includes a transmission delay associated with transmission of the time reference event from the separate component to the wireless data acquisition module; and        adjusting a system clock of the wireless data acquisition module, based on the delay value, to synchronize the system clock in relation to the time reference event (i.e. the reference clock).        
In a particular embodiment described in this document, the GPS receiver is powered on at a first instance, in order to discipline the system clock of the wireless data acquisition module to the GPS time signal received (and relating to a first reference clock), and subsequently powered off. Subsequently, the system clock control of the wireless data acquisition module employ time reference events received from radio events (and relating to a second reference clock) to continue to discipline the system clock, such that the GPS receiver may remain powered down to conserve battery capacity.
The adjusted clock of a first wireless data acquisition module may be used to generate an outgoing reference signal (above second reference clock) that is then broadcast to a second wireless data acquisition module which may in turn perform a corresponding process to the one described above. This allows to propagate time reference events throughout the system such that all wireless data acquisition modules within the system have synchronous clocks.
A second known solution is described in WO2005029131A2, entitled “Single station wireless seismic data acquisition method and apparatus”. In one embodiment, the wireless acquisition unit (also referred to as “wireless sensor station”) includes a GPS receiver and associated antenna. The GPS receiver in this embodiment is shown coupled to a processor and a clock circuit to provide location parameters for correlating seismic information and for synchronizing data acquisition. Alternatively, location parameters can be transmitted to and stored in a central controller and synchronization may be accomplished by sending signals over a VHF/UHF radio link independent of the GPS.
A third known solution is described in US20050047275A1, entitled “Synchronization and positioning of seismic data acquisition systems”. To facilitate solution of the synchronization problem, a network master clock and one or more additional high precision clocks are added to the network of wireless acquisition units (also referred to as “remote acquisition modules (RAMs)”). Synchronization of the network is done in two stages, first synchronization of the high precision clocks throughout the network, and subsequently, synchronization of the remainder of the clocks. The high precision clocks can be located internally or externally to the remote acquisition modules (RAMs) and Line tap units (LTUs). In a particular embodiment, the high precision clock possesses an oscillator of lower precision, such as 0.5 PPM, but in this case the high precision clock module relies on the GPS receiver or radio beacon signals to attain high precision. In this particular embodiment, the highly precise GPS time signals or radio beacon signals are used to continually correct the drift of the less precise clock, and in this way the high precision clock module does achieve high precision. There are two types of remote acquisition modules: those with and those without a high precision clock module. Three different mechanisms to synchronize the high precision clock are described: 1) synchronizing before deployment, 2) synchronizing after deployment through direct transmission, and 3) synchronizing after deployment through repeated synchronization transmissions. For the remote acquisition modules without a high precision clock module, they are synchronized to the remote acquisition modules with a high precision clock module: each remote acquisition module receives the synchronization signal from a neighboring remote acquisition module one physical side and rebroadcasts the synchronization signal to another neighboring remote acquisition module on its other physical side. In this way, the synchronizing signal travels to all the remote acquisition modules connected to the network. When a remote acquisition module with a high precision clock receives the synchronizing signal, it corrects it before rebroadcasting it.
However, none of the aforesaid first, second and third known solutions addresses the problem of the choice of the clock source (i.e. of the reference clock) when a given data acquisition module can receive signals from several reference clocks, e.g.:                signals from a receiver (GNSS receiver or synchronization beacon receiver) comprised in or connected to the given data acquisition module, and/or        signals from one or several other data acquisition modules (as is the case notably in a wireless multi-hop network).        
This problem is critical because conditions vary over time and the given data acquisition module does not always receive the same reference clocks. For example, a GNSS receiver can have no fix because of harsh environment, and therefore it can not provide GNSS time signals to the given data acquisition module.
This problem is critical also because there is a need, in a data acquisition system (notably a wireless seismic data acquisition system) based on e.g. 50 acquisition modules or more, for several Master Clocks. A Master Clock is defined as an acquisition module, synchronized by its internal GNSS receiver (or its synchronization beacon receiver), and providing time references to other acquisition modules in the system.